1. Technical Field
The invention relates generally to roller cone drill bits for drilling earth formations, and more specifically to roller cone drill bit designs.
2. Background Art
Roller cone drill bits and fixed cutter bits are commonly used in the oil and gas industry for drilling wells. FIG. 1 shows one example of a roller cone drill bit used in a conventional drilling system for drilling a well bore in an earth formation. The drilling system includes a drilling rig 10 used to turn a drill string 12 which extends downward into the well bore 14. Connected to the end of the drill string 12 is a roller cone-type drill bit 20, shown in further detail in FIG. 2.
Referring to FIG. 2, roller cone drill bits 20 typically comprise a bit body 22 having an externally threaded connection at one end 24, and a plurality of roller cones 26 (usually three as shown) attached at the other end of the bit body 22. The cones 26 are able to rotate with respect to the bit body 22. Disposed on each of the cones 26 of the bit 20 is a plurality of cutting elements 28 typically arranged in rows about the surface of each cone 26.
The cutting elements 28 on a roller cone 26 may include primary cutting elements, gage cutting elements, and ridge cutting elements. Primary cutting elements are the cutting elements arranged on the surface of the cone such that they contact the bottomhole surface as the bit is rotated to cut through the formation. Gage cutting elements are the cutting elements arranged on the surface of the cone to scrape the side wall of the hole to maintain a desired diameter of the hole as the formation is drilled. Ridge cutting elements are miniature cutting elements typically located between primary cutting elements to cut formation ridges that may pass between the primary cutting elements to protect the cones and minimize wear on the cones due to contact with the formation. The cutting elements 28 may be tungsten carbide inserts, superhard inserts, such as polycrystalline diamond compacts, or milled steel teeth with or without hardface coating.
Significant expense is involved in the design and manufacture of drill bits to produce bits which have increased drilling efficiency and longevity. For more simple bit designs, such as those for fixed cutter bits, models have been developed and used to design and analyze bit configurations which exhibit balanced forces on the cutting elements of the bit during drilling. Fixed cutter bits designed using these models have been shown to provide faster penetration and long life.
Roller cone bits are more complex than fixed cutter bits, in that the cutting surfaces of the bit are disposed on roller cones, wherein each roller cone independently rotates relative to the rotation of the bit body about an axis oblique to the axis of the bit body. Because the cones rotate independently of each other, the rotational speed of each cone of the bit can be different from the rotation speed of the other cones. The rotation speed for each cone of a bit can be determined from the rotational speed of the bit and the effective radius of the “drive row” of the cone. The effective radius of the drive row is generally related to the radial extent of the cutting elements that extend axially the farthest from the axis of rotation of the cone, these cutting elements generally being located on a so-called “drive row”. Adding to the complexity of roller cone bit designs, the cutting elements disposed on the cones of the roller cone bit deform the earth formation by a combination of compressive fracturing and shearing. Additionally, most modern roller cone bit designs have cutting elements arranged on each cone so that cutting elements on adjacent cones intermesh between the adjacent cones, as shown for example in FIG. 3A and further detailed in U.S. Pat. No. 5,372,210 issued to Harrell. Intermeshing of the cutting elements on roller cone bits is desirable to enable high insert protrusion to achieve good rates of penetration while preserving the longevity of the bit. However, intermeshing cutting elements on roller cone bits substantially constrains cutting element layout on the bit, thereby further complicating the designing of roller cone drill bits.
Because of the complexity of roller cone bit designs, accurate models of roller cone bits have not been widely developed or used to design roller cone bits. Instead, roller cone bits have largely been developed through trial and error. For example, if cutting elements on one cone of a prior art bit wore down faster that the cutting elements on another cone of the bit, a new bit design would be developed by simply adding more cutting elements to the faster worn cone in hopes of reducing the wear of each cutting element on that cone. Trial and error methods for designing roller cone bits have led to roller cone bits which have an imbalanced distribution of force on the bit. This is especially true for roller cone bits having cutting elements arranged to intermesh between adjacent cones.
Using a method for simulating the drilling performance of roller cone bits drilling earth formations, described in a patent application filed in the United States on Mar. 13, 2000, entitled “Method for Simulating the Drilling of Roller Cone Drill Bits and its Application to Roller Cone Drill Bit Design and Performance” and assigned to the assignee of this invention, prior art roller cone bits were analyzed and found to typically unequally distribute the axial force on the bit between the cones, such that the axial forces on two cones differ by more than 200%. Such an unequal distribution of force between the cones results in an unequal distribution of stress, strain, wear, and premature failure of the cone or cones carrying the largest load(s) during drilling. Additionally, prior art roller cone bits typically have significant imbalances in the distribution of the volume of formation cut between the cones. In such prior art bits, the volume of formation cut by each cone, typically, differs by more than 75%, wherein the volume cut by one cone was 75% more than the volume of formation cut by each of the other cones on the bit. Prior art bits also have substantial imbalance between the amount of work performed by each of the cones on the bit.
Additionally, prior art bits with cutting elements arranged to intermesh between adjacent cones have significant differences in the number of cutting elements on each cone in contact with the formation during drilling. Prior art bits also typically have large differences in the projected area of cutting elements in contact with formation on each cone, and in the depths of penetration achieved by the cutting elements on each cone. As a result, the projection area of cutting element contact for each cone greatly differs in typical prior art bit designs. Additionally, the cutting elements on each cone of prior art bits typically achieve unequal depths of penetration for each cone. In some prior art designs, the unequal cutting element penetration depth between the cones is partially due to the bottomhole profile formed by the bit during drilling. Additionally for typical prior art bits, the axial force on the bit is distributed in a multi-modal profile and the forces on corresponding rows of each cone may significantly differ. Further, prior art bits often have cutting elements arranged about the surface of each cone such that forces acting on corresponding cutting elements on each cone significantly differ. Using drill bits which have multi-modal force distributions, or an unequal distribution of force between corresponding rows of the cones or corresponding cutting elements of the cones may result in a bottomhole profile formed by the bit that is multi-modal which may contribute to the unequal cutting element penetration depth and an imbalanced distribution of force on the bit between the cones.
One example of a prior art bit considered effective in the drilling wells is shown in FIGS. 3A-3D. This drill bit comprises a bit body 100 and three roller cones 110 attached thereto, such that each roller cone 110 is able to rotate with respect to the bit body 100 about an axis oblique to the bit body 100. Disposed on each of the cones 110 is a plurality of cutting elements 112 for cutting into an earth formation. The cutting elements are arranged about the surface of each cone in generally circular, concentric rows substantially concentric with the axis of rotation of the respective cone, as illustrated in FIG. 3C. In FIG. 3A, the profiles of each row of cutting elements on each cone are shown in relation to each other to show the intermeshing of the cutting elements between adjacent cones. In this example, the cutting elements comprise milled steel teeth with hardface coating applied thereon. This type of drill bit is commonly referred to as a “milled tooth” bit.
As is typical for milled tooth roller cone bits, the teeth are arranged in three rows 114a, 114b, and 114c on the first cone 114, two rows 116a and 116b on the second cone 116, and two rows 118a and 118b on the third cone 118. At least one row of teeth on each cone is arranged to intermesh with a row of teeth on an adjacent cone. The first row 114a of the first cone 114 is located at the apex of the cone and is typically referred to as the spearpoint of the bit.
The drilling performance of this prior art bit was simulated and analyzed using the method described in the previously referred to patent application (filed in the United States on Mar. 13, 2000, entitled “Method for Simulating the Drilling of Roller Cone Drill Bits and its Application to Roller Cone Drill Bit Design and Performance” and assigned to the assignee of this invention). From this analysis, it was found that the prior art bit has unbalanced axial force between the cones, wherein the axial force on the bit during drilling was distributed between the first 114, second 116, and third 118 cones in the ratio of 2.91:1.67:1, respectively. Thus, the axial force on the first cone during drilling, on average, was approximately three times the axial force on the third cone. Additionally, this prior art bit was found to exhibit rock cutting volume ratios for the first 114, second 116 and third 118 cones of 1.84:1.03:1, respectively, wherein the first cone 114 was found to cut over 80% more rock than the third cone 118.
In designing roller cone bits, ideally the cutting elements are disposed on the bit such that the same number of cutting elements on each cone contacts the formation at each point in time throughout drilling. However, in practical bits, the number of cutting elements on each cone which contacts the formation differs at each point in time throughout drilling. For example, at one instant in time a cone may have three cutting elements in contact with a formation. At another instant in time the same cone may have two cutting elements in contact with the formation. At a third instant in time the cone may have four cutting elements in contact with the formation. Therefore, in order to determine whether the number of cutting elements on the bit contacting a formation is equally distributed between the cones, the fraction of the total time that each number of cutting elements on each cone instantaneously contacts the formation must be compared. In an analysis of typical tri-cone prior art bits, it was found that the distribution of the time a number of cutting elements on each cone contacts a formation during drilling significantly differed for each cone.
One example of a distribution of contact for a prior art bit is shown in FIGS. 8A-8D. The drill bit in this example was a tri-cone bit with milled steel teeth, similar to the drill bit shown in FIGS. 3A-3D. FIG. 8A shows a distribution of the time that each of a number of cutting elements contacts the earth formation during drilling for the entire bit. FIG. 8B-8C each show a distribution of the time that each of a number of cutting elements on each cone contacts the earth formation during drilling. From FIGS. 8A-8C, it can be observed that the distributions of contact for each cone are significantly different. For example, the second cone has two or fewer cutting elements in contact with the formation the majority of the time, while the first and third cones have three or more cutting elements in contact the majority of the time. In particular, the first, second and third cones have three or more cutting elements in contact with the formation 70%, 45%, and 55% of the time, respectively. Thus, the contribution of each cone significantly differs. Further, it can be seen that the greatest difference between the fraction of time a given number of cutting elements on each cone contacts the earth formation during drilling is approximately 27%, wherein the first cone has two cutting elements in contact with the formation approximately 16% of the time, while the second cone has two cutting elements in contact with the formation approximately 43% of the time. Additionally, it can be determined from these distributions that the first cone has an average of about 3.3 cutting elements in contact with the formation during drilling, while the second and third cones average about 2.35 and 2.52 cutting elements in contact during drilling, respectively. Thus, the contribution of the first cone to the number of cutting elements in contact with the formation is greater than the contribution of each of the other two cones. The largest difference in the average number of cutting elements in contact with the formation between cones is approximately 0.95 cutting elements. Thus, on average, the first cone has one more cutting element in contact with the formation during drilling than the second cone, and almost one more cutting element in contact than cone three. While this average difference in the number of cutting elements contacting the formation is only one cutting element, such an imbalance in the distribution of contact between the cones, may result in an imbalanced distribution of force, stress, strain, and wear between the cones, which may lead to the premature failure of the bit. Thus, it is desirable to design a bit having intermeshing cutting elements between the cones, wherein the average number of cutting elements contacting the formation is substantially the same for each cone, so that wear on the bit is more equally distributed between the cones, potentially increasing the effectiveness and longevity of the cones and the bit.